1. Field of the Invention
The invention relates to a flat panel display apparatus and a method of manufacturing thereof and more particularly the invention is suitable when it is applied to a flat panel display using a field emission type cathode array.
2. Description of the Prior Art
Hitherto, as a flat panel display using a field emission type cathode array comprising microtips of the size of a micron order, a display as shown in FIG. 1 is known.
As shown in FIG. 1, in the conventional flat panel display, a silicon dioxide (SiO.sub.2) film 102 having cavities 102a is formed on a conductive flat silicon (Si) substrate 101. Gate electrodes 103 made of molybdenum (Mo), niobium (Nb), or the like are formed on the SiO.sub.2 film 102 in the peripheral portions of the cavities 102a. A cathode 104 made Of Mo or the like is formed on the Si substrate 101 in the cavity 102a. A fluorescent screen in which a fluorescent material 106 is formed on the flat glass plate 105 is arranged so as to face in parallel with the Si substrate 101 on which the cathode array is formed. The space between the fluorescent screen and the Si substrate 101 is sealed in a state in which it is held in vacuum.
In recent years, a request to realize a large screen of the flat panel display is becoming strong. The above conventional flat panel display, however, has a structure in which a differential pressure between the atmospheric pressure and the vacuum is held by only the glass plate 105 on which the fluorescent material 106 is formed. It is, accordingly, difficult to simply enlarge the screen from a viewpoint of the strength of the glass plate 105.
To solve the above problem, there is considered a method of realizing a large screen by forming the screen into a spherical shape as shown in FIG. 2 as in the case of the cathode ray tube of an ordinary television receiver. When such a structure is used, however, the formation of a portion in which a distance between the cathode array and the fluorescent screen is large cannot be avoided. In the above conventional flat panel display, however, it is necessary to closely arrange the cathode array and the fluorescent screen in terms of the operation principle. Therefore, when the screen is simply formed in a spherical shape as mentioned above, a trouble occurs in the operation of the flat panel display. To prevent it, there is also considered a method whereby the Si substrate is also formed in a spherical shape and the cathode array is formed on the Si substrate. It is, however, extremely difficult to realize such a structure from a viewpoint of the manufacturing processes.
Therefore, in the flat panel display shown in FIG. 1, there is considered a method whereby pillars are formed at regular intervals between the glass plate 105 and the Si substrate 101 and the differential of pressure between the atmospheric pressure and the vacuum is held by the pillars. When such a structure is used, however, there are problems such that not only the manufacturing processes become complicated but also the cathode 104 cannot be formed on the Si substrate 101 in the portion of the pillar.
From the above reasons, it is so far difficult to realize a large screen of the flat panel display using the field emission type cathode array.
On the other hand, as a method of manufacturing a flat panel display using a field emission type cathode array by microtips of the size of a micron order a method as shown in FIGS. 3A to 3E is known. According to the manufacturing method, as shown
in FIG. 3A, an SiO.sub.2 film 102 is first formed on a conductive Si substrate 101 by, for instance, a thermal oxidation method, a CVD method or a sputtering method. After that, a metal film 107 made of, for example, Mo film, a Nb film, or the like to form gate electrodes is formed onto the SiO.sub.2 film 102 by, e.g., a sputtering method or an electron beam evaporation depositing method. Subsequently, a resist pattern 108 having shapes corresponding to the gate electrodes to be formed are formed onto the metal film 107 by a lithography;
The metal film 107 is subsequently etched by a wet etching method or a dry etching method by using the resist pattern 108 as a mask, thereby forming gate electrodes 103 as shown in FIG. 3B. After that, the SiO.sub.2 film 102 is etched by a wet etching method or a dry etching method by using the resist pattern 108 and the gate electrodes 103 as masks, thereby forming cavities 102a.
After the resist pattern 108 was removed, as shown in FIG. 3C, an oblique evaporation deposition is executed to the substrate surface by an electron beam evaporation depositing method from the direction which is inclined by a predetermined angle to the substrate surface, thereby forming a peeling-off layer 109 made of, e.g., aluminium (Al) or nickel (Ni) onto the gate electrodes 103. After that, for instance, Mo as a material to form cathodes is evaporation deposited onto the substrate surface by an electron beam evaporation depositing method from the direction perpendicular to the substrate surface. Thus, cathodes (emitters) 104 comprising microtips are formed onto the Si substrate 101 in the cavities 102a. Reference numeral 110 denotes a metal film which has been evaporation deposited onto the peeling-off layer 109.
The peeling-off layer 109 is subsequently removed by a lift-off method together with the metal film 110 formed On the peeling-off layer 109, so that a state shown in FIG. 3D is obtained. After that, as shown in FIG. 3E, a screen in which a fluorescent material 106 is formed on a glass plate 105 serving as a display screen is arranged so as to face the Si substrate 101 on which the cathode array is formed in a manner such that the fluorescent material 106 is positioned on the inside. The space between such a screen and the Si substrate 101 is sealed in a state in which it is held in vacuum. A desired flat panel display is consequently completed.
Upon operation of the flat panel display, a negative voltage of, e.g., about -50 V is applied to each cathode 104.
In the foregoing conventional manufacturing method of the flat panel display, it is extremely difficult to align all of the radii of curvatures of the tips of a number of (for instance, tens of thousand) cathodes 104 which are simultaneously formed by an evaporation depositing method, and the occurrence of a slight variation in the radii of curvatures of the tips of the cathodes 104 can hardly be avoided.
On the other hand, as shown in FIG. 4, there is generally a correlation between the radius of curvature of the tip of the cathode and an allowable applied voltage to the cathode. In FIG. 4, Vmin denotes a minimum voltage (absolute value) at which a current emission can be performed and Vmax indicates a maximum voltage (absolute value) at which a current emission can be executed without causing a discharge. As will be understood from FIG. 4, as a radius of curvature of the tip of the cathode increases, a voltage at which the current emission can be performed rises. Therefore, if only one cathode whose radius of curvature of the tip is smaller than those of the other cathodes exists, for instance, among tens of thousand cathodes when a negative voltage is gradually applied to those cathodes whose radius of curvature of the tip is smaller than those of the other cathodes. There is a problem such that when the current emission starts from the other cathodes, the voltage exceeds the allowable applied voltage, those cathodes discharge, the tips are rounded, and the current emission stops.
To solve the above problem, a method whereby a resistor is provided between each cathode and a power source to thereby prevent the occurrence of the emission of a predetermined current or higher has also been proposed. There is a problem such that the above method is extremely difficult from a viewpoint of the manufacturing processes.